This invention relates to optical devices such as, for example, optical filters or external cavity lasers.
Optical filters arc generally passive optical elements having a wavelength dependent transmission characteristic.
It is known to construct optical filters using dual-core, substantially lossless optical fibres. Dual-core fibres, as the name suggests, are optical fibrers having two cores in close proximity. Light injected into one core cross-couples periodically into the adjacent core via evanescent field interaction. The cross-coupling strength and the power evolution in each core depend on the geometrical parameters of the fibre such as the core size and core-to-core separation, the optical parameters of the fibre such as the fibre numerical aperture (NA) and cutoff wavelength, and the optical wavelength in use.
In previous optical filters using dual core optical fibres, an optical signal is launched in one of the cores (the input) and emerges from the same or the adjacent core (the output) depending on the length of the fibre used in the filter. Filtering is achieved by the wavelength dependence of the coupling between the two fibres, denoted by the coupling coefficient C.sub.S.
Considering each input signal separately, the optical power in that signal is periodically transferred between the two cores of a dual-core optical fibre. In fact, the transfer of power between the two cores follows a substantially sinusoidal pattern, oscillating between a peak where all or most of the power is in one core, and a peak of opposite polarity where all or most of the power is in the other core. The period of this oscillation (the cross-coupling beat length) is of the order of a few millimeters (mm).
At any point along the dual core fibre, therefore, the relative proportions of the input light in each of the two fibres depends on the current phase of the periodic transfer of light of that wavelength. By selecting a fibre of a particular length, the transfer phase of light of one wavelength can be selected so that the light of that wavelength is substantially all in one core, with light of another (undesired) wavelength being all in the other core. The filter output is taken from only one core (the core in which the desired wavelength is at a maximum at the end of the filter).
Fibre filters of this type are very sensitive to environmental variations, since their operation depends heavily on the fibre length and the phasing of the transfer between the two cores.